Process for producing a calcium phosphate reactant, reactant obtained and use thereof in the purification of liquid effluents

ABSTRACT

A process for producing a calcium phosphate reactant, according to which: in a first step, use is made of a source of calcium and a source of phosphate ions in water, in a molar ratio that is adjusted so as to obtain a Ca/P molar ratio of between 0.5 and 1.6, and the source of calcium is reacted with the phosphate ions at a pH of between 2 and 8, in order to obtain a suspension (A) of calcium phosphate, and in a second step, added to the suspension (A) are an alkaline compound comprising hydroxide ions in order to set a pH of more than 8 and an additional source of calcium in order to obtain a suspension (B) of calcium phosphate reactant having a Ca/P molar ratio of more than 1.6. 
     A calcium phosphate reactant obtainable by such a process.

TECHNICAL FIELD

This application claims priority to French application No. 1454383 filedon 16 May 2014, the whole content of this application being incorporatedherein by reference for all purposes.

The invention relates to a process for producing a calcium phosphatereactant. It also relates to the reactant obtained and the use thereoffor treating liquid effluents or for treating substances contaminated byheavy metals.

BACKGROUND ART

The problems posed by the impact of heavy metals in the environment arewell known. Numerous industrial processes release liquid or gaseouseffluents that are heavily loaded with heavy metals, in particular heavymetal soluble salts, such as cationic form salts. The expression “heavymetals” is understood to mean metals whose density is at least equal to5 g/cm³, and also beryllium, arsenic, selenium, and antimony, inaccordance with the generally accepted definition (Heavy Metals inWastewater and Sludge Treatment Processes; Vol I, CRC Press Inc; 1987;page 2). Lead or cadmium are particularly significant examples, giventheir harmful effect on the human body. Nickel is another examplethereof due to its allergenic effect.

It is thus useful to be able to have reactants capable of absorbing andretaining large amounts of heavy metals for treating industrial liquideffluents or wastewaters originating from treatment plants before therelease thereof into the natural environment, or even the treatment ofnatural aquifer waters, some of which are naturally loaded with heavymetals.

Another example of a problem linked to heavy metals is the combustion ofwastes, especially household waste, producing a vaporization of heavymetals, these vapours being entrained in the combustion flue gases. Toavoid contaminating the environment, it is necessary to provide flue gastreatment processes capable of carrying out effective scrubbing of heavymetals. The toxic substances removed from the flue gas when it ispurified are found in a residue which itself must often be treatedbefore being valorized or discharged. Indeed such residue, whichcontains the heavy metals removed from the flue gas, when subjected, forexample, to the action of rain that is acidic when discharged,frequently releases some of the heavy metals that it contains into theenvironment. This can then cause pollution of the subsoil. It istherefore essential that the heavy metals be immobilized in the residue.

FR2912396, describes a process for producing a calcium phosphatereactant, according to which a source of calcium carbonate and a sourceof phosphate ions in water in a weight ratio that is controlled so as toobtain a Ca/P molar ratio of between 1.4 and 1.8 are reacted, at a pH ofbetween 5 and 10, preferably between 7 and 8, with controlled stirringin order to give rise to the appearance of a calcium phosphate gelhaving a viscosity of at least 200 centipoise (mPa·s). The calciumphosphate reactant obtained by such a process is in the form ofparticles having a high BET specific surface area of about 130 m²/g.

However, the particles of such a reactant have the drawback of beingparticularly fine, with weight-average diameters of less than 10 μm,which poses usage problems in sludge blanket treatment of liquideffluents since such particles are easily entrained out of the reactorseven at low flow rates of the effluents in decantors, which may giverise to losses of reactants. Moreover, such particles release largeamounts of phosphates during their use, which has to be avoided in orderto limit the risks of eutrophication of watercourses, since phosphorusis a natural fertilizer that promotes the growth of algae.

DE10330689 discloses granules of hydroxyapatite with particle sizebetween 1 and 5 mm. The document is silent on BET specific surface ofsuch granules. The document discloses a method for purifying a liquideffluent comprising fluor or nickel, in which said granules are put intocontact with said liquid effluent in a column, during a time sufficientso that granules adsorb at least part of fluor or nickel. Though,particle sizes of such granules are too important to be correctly usedin mixed reactors or in sludge blanket decantors.

SUMMARY OF INVENTION

The invention aims to propose an improved process for producing acalcium phosphate reactant, that makes it possible to obtain aneffective reactant for immobilizing heavy metals, in particular insludge blanket effluent treatment units, and that limits the emissionsof phosphates both during the production of the reactant and during theuse of the reactant for fixing heavy metals.

Consequently, the invention relates to a process for producing a calciumphosphate reactant, according to which:

-   -   in a first step, use is made of a source of calcium and a source        of phosphate ions in water in a molar ratio that is adjusted so        as to obtain a Ca/P molar ratio of between 0.5 and 1.6, and the        source of calcium is reacted with the phosphate ions at a pH of        between 2 and 8, in order to obtain a suspension (A) of calcium        phosphate, and    -   in a second step, added to the suspension (A) are an alkaline        compound comprising hydroxide ions in order to set a pH of more        than 8, preferably of more than 8.5, preferably of at least 9,        or of at least 10, and an additional source of calcium in order        to obtain a suspension (B) of calcium phosphate reactant having        a Ca/P molar ratio of greater than 1.6, preferably greater than        1.65.

It has been found that the reactant obtained by the process according tothe invention has novel properties. It consists of particles, the meandiameter D50 of which is greater than 10 μm, in general greater than 20μm, or even greater than 50 μm. However, this mean diameter of thereactant is in general preferably less than 200 μm, or even less than150 μm. This makes it possible to limit the stirring powers to preventthe particles of reactant from settling too easily in a stirred reactoror from needing high velocities of flue gases to be treated at theinjection points of the reactant for the treatment of the flue gases.

Moreover the reactant from the invention, when made in a first step atlow temperature (less than 40° C.), and the second step made at highertemperature (more than 40° C. or of at least 50° C., or of at least 60°C.), has shown particularly high specific surface (at least 110 m²/g, ormore) and particular high adsorbtion capacity of metals, and organicmolecules.

For comparison, for instance natural apatite found in Maroco havespecific surfaces of about 16 m²/g.

The reactant particles of present invention are composed of calciumphosphate, the structure of which is intermediate between tricalciumphosphate and calcium phosphate hydroxyapatite. These particles thenevolve very rapidly towards an apatite structure. Such reactantparticles are composed on their surface of plate like crystallites, ofthickness of a few nano-meters (nm).

It has also emerged that the reactant according to the present inventionhad a remarkably low solubility of the phosphates contained in thereactant particles. Without wishing to be bound by one theoreticalexplanation, the inventors believe that this behaviour stems from thefact that during the second step the addition of an alkaline compoundcomprising hydroxide ions in order to set a pH of more than 8, or of atleast 8.5, or even of at least 9, or of at least 10, makes it possibleto form, at the surface of the particles of the calcium phosphatereactant, mixed calcium hydroxide and calcium phosphate compounds thatactually limit the solubility of the phosphates, for example duringtreatment of wastewaters having a pH close to neutrality.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 Is a scanning electron microscope (SEM) picture of reactantparticles according the invention at magnification of 1 000.

FIG. 2 Is a scanning electron microscope (SEM) picture of reactantparticle according DE10330689 document at same magnification as FIG. 1(of 1 000).

FIG. 3 Is a scanning electron microscope (SEM) picture of reactantparticles according the invention at magnification of 80 000, showingthe surface of reactant particle covered with plate-like crystallites.

FIG. 4 Is a scanning electron microscope (SEM) picture of reactantparticles according DE10330689 document at same magnification as FIG. 3(of 80 000), showing the surface of reactant particle covered withneedles-like crystallites.

DETAILED DESCRIPTION OF INVENTION

The particles of calcium phosphate reactant according to the inventionin general comprise at least 50% calcium phosphate, advantageously atleast 60% and more advantageously still at least 80% calcium phosphate.The balance to 100% in general comprises: water, of the order of from 0to 20%, advantageously from 5% to 20%, calcium carbonate from 0 to 20%,advantageously from 5% to 20%, calcium hydroxide from 0 to 20%,advantageously from 1% to 4%. The particles of calcium phosphatereactant may additionally contain residual compounds originating fromthe use of the raw materials in the process such as: CaCl₂, Ca(NO₃)₂,sands or clays; these constituents are in general less than 5% byweight, advantageously less than 2% by weight.

The term “apatite” denotes a family of mineral compounds, the chemicalformula of which can be written in the following general form:

Me₁₀(XO₄)₆Y₂

In this formula, Me generally represents a divalent cation (Me²⁺), XO₄ atrivalent anionic group (XO₄ ³⁻) and Y a monovalent anion (Y⁻).

Calcium phosphate hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ crystallizes in thespace group of the hexagonal system. This structure consists of aclose-packed quasi-hexagonal stack of XO₄ groups, forming two types ofparallel tunnels.

The existence of these tunnels gives apatites chemical properties akinto those of zeolites. Vacancies may also be created either by thedeparture of anions and cations, or by the presence of cations or anionsof different valency. Apatites therefore appear to be particularlystable structures which may tolerate large gaps in their composition.

Hydroxyapatite should not be confused with tricalcium phosphate (TCP),which has a similar weight composition: Ca₃(PO₄)₂. The Ca/P molar ratioof TCP is 1.5 whereas it is 1.667 for hydroxyapatite. Industrialapatites sold as food additives or mineral fillers are, as a generalrule, variable mixtures of TCP and hydroxyapatite.

Other salts of calcium and phosphate, including TCP, do not have thesame properties as hydroxyapatite. Although TCP can also react withheavy metals, hydroxyapatite is more advantageous as it can enclosemetals in the form of an insoluble, and therefore relatively inert,matrix.

In the present invention, the source of calcium advantageously comprisescalcium carbonate, or calcium oxide, or calcium hydroxide, or calciumchloride, or calcium nitrate, or calcium acetate. The source of calciumis more advantageously a limestone, or a mixture of limestone andcalcium oxide or hydroxide. More advantageously, the source of calciumis in the form of powder or aqueous suspension of powder and the powderis selected from: calcium carbonate, calcium oxide, calcium hydroxide,or a mixture thereof, and the powder has a mean particle size of lessthan 300 μm.

It is advantageous for the source of calcium selected from calciumcarbonate, calcium oxide, calcium hydroxide or mixtures thereof to be inthe form of a powder or aqueous suspension of powder, and to have asmall particle size. In one recommended variant, the mean diameter ofthe particles of the powder is less than 300 μm, advantageously lessthan 200 μm and preferably less than 100 μm. The mean diameter inquestion is the D50, that is to say the diameter such that 50% by weightof the particles have a diameter less than said value.

In the present invention, various sources of phosphate ions may be used,in particular:

-   -   phosphoric acid,    -   or a dihydrogen phosphate salt such as a sodium, potassium or        ammonium dihydrogen phosphate salt, preferably a sodium        dihydrogen phosphate salt,    -   or a monohydrogen phosphate salt such as a sodium, potassium or        ammonium monohydrogen phosphate salt, preferably a sodium        monohydrogen phosphate salt.

In the present invention, phosphoric acid is preferred due to itsgreater availability and lower cost compared to dihydrogen andmonohydrogen phosphate salts.

In the process according to the invention, in the first step the Ca/Pmolar ratio is in particular:

-   -   between 0.50 and 1.35, preferably between 0.70 and 1.30,    -   or: between 1.4 and 1.6, preferably between 1.4 and 1.5.

During the first step where calcium and phosphate are used in a Ca/Pmolar ratio of between 0.5 and 1.6 and where they are reacted at a pHbetween 2 and 8, the compounds formed in the suspension (A) are amixture of monocalcium phosphate (MCP) having the weight formulaCa(H₂PO₄)₂, of dicalcium phosphate (DCP) having the weight formulaCaHPO₄, or the hydrate thereof, brushite, having the weight formulaCaHPO₄.2H₂O, and of octacalcium having the weight formulaCa₈H₂(PO₄)₆.6.5H₂O. The Ca/P molar ratios are respectively for thesecompounds: 0.5 (MCP), 1.0 (DCP and brushite), 1.33 (octacalcium).

In order to promote, in the first step, the formation of MCP and DCP, aCa/P ratio of between 0.50 and 1.35, preferably between 0.7 and 1.30, isfavoured. This Ca/P molar ratio is particularly advantageous when thesource of calcium from the first step comprises calcium carbonate, andthe source of phosphate is phosphoric acid (H₃PO₄) or is a dihydrogenphosphate salt such as a sodium or potassium or ammonium salt.Specifically, this makes it possible to obtain a rapid attack of thecalcium carbonate and a rapid degassing of the CO₂. In addition tocalcium carbonate, the source of calcium may comprise calcium oxide, orcalcium hydroxide, or calcium chloride, or calcium nitrate, or calciumacetate.

In order to promote, in the first step, the formation of DCP andoctacalcium, a Ca/P ratio of between 1.4 and 1.6, preferably between 1.4and 1.5, is favoured. This molar ratio is advantageous when use is madeof a source of calcium having less than 30% by weight of carbonate, suchas preferably: calcium oxide, or calcium hydroxide, or calcium chloride,or calcium nitrate, or calcium acetate.

In the present invention, in the first step, the source of calcium andthe phosphate ions are in general reacted for at least 0.1 hour,preferably at least 0.5 hour. It is not useful to react them overexcessively long durations. Advantageously, the source of calcium andthe phosphate ions are reacted for at most 4 hours, more advantageouslyat most 2 hours, or even at most 1 hour. For example, a duration of 1hour at pH 5 already enables a good reaction of the calcium and thephosphate ions, and makes it possible to sufficiently release the CO₂when a source of calcium comprising calcium carbonate is used, beforemoving on to the second step.

In the present invention, in the second step, the suspension (B) ofcalcium phosphate reactant in general has a Ca/P molar ratio of at most5, preferably of at most 3, more preferably still of at most 2.

In the present invention, it is advantageous, in the second step, forthe alkaline compound used, that comprises hydroxide ions, to be sodiumhydroxide and/or calcium hydroxide.

In the process according to the invention, it is particularlyadvantageous for the additional source of calcium to be selected fromcalcium chloride, calcium nitrate, or calcium acetate, preferablycalcium chloride, and for it to be added in addition to the alkalinecompound, in order to finely adjust the Ca/P ratio and in order to limitthe concentration of phosphorus element in the aqueous solution (C) ofthe suspension (B) to at most 5 mmol, advantageously to at most 0.5mmol, more advantageously to at most 0.05 mmol of phosphorus element perlitre of aqueous solution (C). Specifically, this makes it possible,coupled with the use of hydroxide ions for setting the pH of the secondstep, to limit the losses of phosphates in the process waters.

In the present invention, in general, the stirring and the density ofsuspension, in the second step and advantageously also in the firststep, are adjusted in order to avoid the appearance of a calciumphosphate gel having a viscosity of at least 200 cps. The viscosity ofthe calcium phosphate reactant suspension (B) in the second step of theprocess of present invention is typically about 10 cps (mPa·s).Specifically, the production of a gel, even in the presence of thesecond step, results in particles of calcium phosphate reactant of smallparticle size being produced, with weight-average D50 values of lessthan 10 μm, which is a disadvantage for certain applications of liquideffluents such as those that use a sludge blanket.

The suspended solids density of the suspension (A) in the first step isin general at most 20%. The suspended solids density of the suspension(B) in the second step is in general at most 15%. The suspended solidsdensity of the suspension (A) and or of the suspension (B) isadvantageously at least 10%. It has been indeed observed that a too lowdensity of suspension decreases the efficacy of the produced reactantparticles in heavy metal adsorption (in particular on Zn, Cu, Ni).Moreover a too low density of suspension induces longer time of waterseparation when decantation or filtration is used in the process.

In the process of present invention, the stirring of the suspensionduring the first and second steps corresponds generally to a stirringdissipated energy in the reactors volume of at least 0.2 and at most 1.5kW/m³, preferably at least 0.5 and at most 1.0 kW/m³.

In a first embodiment of the present invention, the first step iscarried out at a temperature of less than 50° C., preferably at most 45°C., or at most 40° C. This makes it possible to obtain a calciumphosphate reactant at the end of the second step in the form ofparticles of large to medium particle size and having a high specificsurface area. The invention relates in particular to a particle ofcalcium phosphate reactant by this first embodiment, comprising at least60% by weight of hydroxyapatite, and having a mean size of at least 30μm, preferably of at least 50 μm and having a specific surface area ofat least 50 m²/g, preferably of at least 110 m²/g, and comprising atleast 2%, preferably at least 5%, and more preferably at least 6% byweight of hydroxide ions.

In a second embodiment of the present invention, the first step iscarried out at a temperature of at least 50° C., preferably of at least55° C., or of at least 60° C. This makes it possible to obtain a calciumphosphate reactant in the second step in the form of particles of smallparticle size and having a lower specific surface area. The inventionrelates in particular to a particle of calcium phosphate reactantobtained by the process according to this second embodiment, comprisingat least 60% by weight of hydroxyapatite, and having a mean size of atmost 30 μm, preferably of at most 20 μm and having a specific surfacearea of at least 15 m²/g, preferably of at least 50 m²/g, and having acontent of hydroxide ions of greater than 2% by weight, preferablygreater than 3.5% by weight, and more preferably greater than 4% byweight.

In the first or second embodiment of the process of the presentinvention, it is advantageous for the second step to be carried out at atemperature of at least 45° C., preferably of at least 55° C., or of atleast 60° C., or of at least 80° C. Specifically, this makes it possibleto rapidly convert the calcium phosphate compounds from the first stepinto the calcium phosphate reactant according to the invention, with agood fixation of the hydroxide ions at the core and at the surface ofthe calcium phosphate reactant, and to more rapidly consume thephosphates from the solution of the suspension (B). Advantageously, thesecond step is carried out at a temperature of at least 45° C.,preferably of at least 55° C., or of at least 60° C., or of at least 80°C., for a duration of at least 0.1 to 0.5 hour. In general, the additionof the alkaline compound comprising hydroxide ions in order to set thepH of the second step, and of the additional source of calcium in orderto obtain a suspension (B) of calcium phosphate reactant having a Ca/Pmolar ratio of greater than 1.6 last no more than 4 hours,advantageously no more than 2.5 h: at higher temperature such as at 50or at 60° C. generally one hour is sufficient, as at 40° C. the alkalinecompound addition to set the pH of the second step is generally longer:and about 2.5 hours are needed. Preferably, the alkaline compoundaddition is stopped when the pH remains at the set value for at least 15minutes. Advantageously, once the additions of hydroxide ions forsetting the pH of the second step, and the addition of the additionalsource of calcium are completed, the suspension (B) is left to cool for1 to 24 hours, preferably at least 10 hours, down to ambienttemperature. This makes it possible to mature the calcium phosphatereactant and to reduce the residues of MCP/DCP or brushite, or ofoctacalcium (precipitated during the first step), into hydroxyapatiteand into calcium phosphate and calcium hydroxide complexes, within thesuspension (B).

Optionally, in the process of the present invention, at the end of thesecond step, the suspension (B) comprises an aqueous solution (C) andcalcium phosphate reactant particles, and

-   -   in a third step, a portion of the aqueous solution (C) is        separated from the suspension (B) in order to obtain an aqueous        suspension (D) comprising at least 18% and at most 50% of        calcium phosphate reactant particles, or in order to obtain a        wet solid (D′) comprising at least 50% and at most 80% of        calcium phosphate reactant particles, or a pulverulent solid        (D″) comprising at least 80% and at most 95% of calcium        phosphate reactant particles and at least 5% and at most 20% of        water.

Consequently, the present invention also relates to an aqueoussuspension (D) comprising at least 25% and at most 50% of calciumphosphate reactant particles obtained by the present process, or to awet solid (D′) comprising at least 50% and at most 80% of calciumphosphate reactant particles obtained by the present process, or apulverulent solid (D″) comprising at least 80% and at most 95% ofcalcium phosphate reactant particles obtained by the present process andat least 5% and at most 20% of water.

The calcium phosphate reactant obtained according to the presentinvention is effective for treating substances contaminated by metallicand/or non-metallic elements, in particular contaminated by heavymetals. Consequently, the present invention also relates to a method forpurifying a substance contaminated by metallic and/or non-metallicelements according to which the substance, such as waters, gases, fluegases, solid residues or soils, is brought into contact with the calciumphosphate reactant obtained according to the process of the presentinvention, in particular with the calcium phosphate reactant of thepresent invention, or with the suspension (D) or the wet solid (D′) orthe pulverulent solid (D″) of the present invention, in order that atleast one portion of the metallic elements of the substance is adsorbedby the calcium phosphate reactant.

In the purification method according to the invention, the contaminatedsubstance may be a flue gas containing metallic and/or non-metallicelements such as As, B, F, Se, and according to which the calciumphosphate reactant, or the aqueous suspension (D) or the wet solid (D′)or the pulverulent solid (D″), is dispersed in the flue gases, the fluegases being at a temperature above 100° C., the resulting mixture thenbeing subjected to a separation in order to obtain a resulting solid anda flue gas partially purified of metallic and/or non-metallic elements.

In the purification method according to the invention, the contaminatedsubstance may be a liquid effluent containing metallic elements such as:Al, Ag, Ba, Be, Ce, Co, Cd, Cu, Cr, Fe, Hg, La, Li, Mo, Ni, Pb, Pd, Rb,Sb, Sn, Th, Ti, U, V, Y, Zn and/or non-metallic elements such as As, B,F, Se, according to which the calcium phosphate reactant or thesuspension of calcium phosphate reactant is mixed into the liquideffluent for a sufficient time so that the calcium phosphate reactantabsorbs at least a portion of the metallic and/or non-metallic elementsand the mixture is subjected to a clarification in order to produce aliquid partially purified of metallic and/or non-metallic elements, onthe one hand, and the calcium phosphate reactant loaded with metallicand/or non-metallic elements that is removed. Preferably, the calciumphosphate reactant is used with the liquid effluent in a contactreactor, such as a sludge blanket reactor or a fluidized bed. Thecontact time between the calcium phosphate reactant and the liquideffluent is in general at least one minute, advantageously at least 15minutes, more advantageously at least 30 minutes, even moreadvantageously at least one hour. In one particularly advantageousembodiment of the invention, the liquid effluent is introduced into asludge blanket contact reactor in which the calcium phosphate reactantis present at a weight concentration of at least 0.5% by weight and ingeneral at most 10% by weight; a liquid is recovered as overflow fromthe sludge blanket reactor; a flocculant is added to the recoveredliquid in order to form a mixture comprising particles of calciumphosphate reactant entrained out of the contact reactor and flocculated;said mixture is then introduced into a settling tank where the mixtureis separated into:

-   -   the liquid partially purified of metallic elements and/or of        non-metallic elements, and said liquid is recovered as overflow        from the settling tank,    -   and into an underflow from the settling tank comprising        flocculated and settled particles of calcium phosphate reactant        recovered as underflow from the settling tank;        and at least one portion of the underflow from the settling tank        containing flocculated and settled particles of calcium        phosphate reactant is recycled to the sludge blanket contact        reactor. The effectiveness of the treatment of metallic elements        and/or non-metallic elements may be monitored by comparing the        concentrations of the elements upstream (in the liquid effluent)        and downstream of the treatment (in the partially treated        liquid), for example by an automatic analyser or by sampling and        analysis. The calcium phosphate reactant charge of the contact        reactor is in general regularly renewed in portions. For        example, by partial purging of the calcium phosphate reactant        loaded with metallic and/or non-metallic element at the        underflow from the settling tank, and by adding fresh calcium        phosphate reactant to the contact reactor. Such a process thus        ensures a “chemical polishing” of the liquid effluents. The        process is particularly advantageous in the case where the        liquid partially purified of metallic elements and/or        non-metallic elements is then treated in a biological treatment        plant producing sewage sludges. This makes it possible to reduce        the concentrations of such elements of said sewage sludges and        to reutilize them, for example in agriculture or in land        development.

In the purification method according to the invention, the contaminatedsubstance may be a solid residue or a soil contaminated by metallicelements such as Al, Ag, Ba, Be, Ce, Co, Cd, Cu, Cr, Fe, Hg, La, Li, Mo,Ni, Pb, Pd, Rb, Sb, Sn, Th, Ti, U, V, Y, Zn and/or non-metallic elementssuch as As, B, F, Se, according to which the calcium phosphate reactant,or the aqueous suspension (D) or the wet solid (D′) or the pulverulentsolid (D″) of calcium phosphate reactant is injected into the solidresidue or the soil in the vicinity of the metallic and/or non-metallicelements for a sufficient time so that the calcium phosphate reactantadsorbs at least a portion of the metallic and/or non-metallic elements.

In particular the present invention relates to the followingembodiments:

Item 1. Process for producing a calcium phosphate reactant, according towhich:

-   -   in a first step, use is made of a source of calcium and a source        of phosphate ions in water, in a molar ratio that is adjusted so        as to obtain a Ca/P molar ratio of between 0.5 and 1.6, and the        source of calcium is reacted with the phosphate ions at a pH of        between 2 and 8, in order to obtain a suspension (A) of calcium        phosphate, and    -   in a second step, added to the suspension (A) are an alkaline        compound comprising hydroxide ions in order to set a pH of more        than 8, preferably of more than 8.5, preferably of at least 9,        or of at least 10, and an additional source of calcium in order        to obtain a suspension (B) of calcium phosphate reactant having        a Ca/P molar ratio of more than 1.6.

Item 2. Process according to the preceding item, in which in the secondstep the pH is set between 8 and 10.5, preferably between 8.5 and 10.

Item 3. Process according to the preceding items, in which the source ofcalcium comprises calcium carbonate, or calcium oxide, or calciumhydroxide, or calcium chloride, or calcium nitrate, or calcium acetate.

Item 4. Process according to any one of the preceding items, in whichthe source of phosphate ions is phosphoric acid.

Item 5. Process according to any one of items 1 to 3, in which thesource of phosphate ions is a dihydrogen phosphate salt such as asodium, potassium or ammonium dihydrogen phosphate salt, preferably asodium dihydrogen phosphate salt,

Item 6. Process according to any one of items 1 to 3, in which thesource of phosphate ions is a monohydrogen phosphate salt such as asodium, potassium or ammonium monohydrogen phosphate salt, preferably asodium monohydrogen phosphate

Item 7. Process according to any one of the preceding items, wherein inthe second step, the suspension (B) of calcium phosphate reactant has aCa/P molar ratio of at most 5, preferably of at most 3, more preferablystill of at most 2.

Item 8. Process according to any one of the preceding items, in which,in the first step, the Ca/P molar ratio is:

-   -   between 0.50 and 1.35, preferably between 0.70 to 1.30,    -   or: between 1.4 and 1.6, preferably between 1.4 and 1.5.

Item 9. Process according to any one of the preceding items, wherein thesource of calcium is in the form of powder or aqueous suspension ofpowder and the powder is selected from: calcium carbonate, calciumoxide, calcium hydroxide, or a mixture thereof, and the powder has amean particle size of less than 300 μm.

Item 10. Process according to any one of the preceding items, in whichthe stirring and the density of suspension, in the second step andadvantageously also in the first step, are adjusted in order to avoidthe appearance of a calcium phosphate gel having a viscosity of at least200 cps.

Item 11. Process according to any one of the preceding items, in which,in the second step, the alkaline compound used that comprises hydroxideions is sodium hydroxide and/or calcium hydroxide and/or magnesiumhydroxide.

Item 12. Process according to any one of the preceding items, in which,in the second step, the additional source of calcium is selected fromcalcium chloride, calcium nitrate, or calcium acetate, preferablycalcium chloride, and is added in addition to the alkaline compound, inorder to finely adjust the Ca/P molar ratio and limit the concentrationof phosphorus element in the aqueous solution (C) of the suspension (B)to at most 5 mmol, advantageously to at most 0.5 mmol, moreadvantageously to at most 0.05 mmol of phosphorus element per litre ofaqueous solution (C).

Item 13. Process according to any one of the preceding items, in whichthe first step is carried out at a temperature of less than 50° C.,preferably of at most 45° C., more preferably of at most 40° C.

Item 14. Process according to any one of items 1 to 12, in which thefirst step is carried out at a temperature of at least 50° C.,preferably of at least 55° C., more preferably of at least 60° C.

Item 15. Process according to any one of the preceding items, in whichthe second step is carried out at a temperature of at least 40° C.,preferably of at least 45° C., more preferably of at least 55° C., evenmore preferably of at least 60° C., or of at least 80° C.

Item 16. Process according to any one of the preceding items wherein atthe end of the second step, the suspension (B) comprises an aqueoussolution (C) and calcium phosphate reactant particles, and

-   -   in a third step, a portion of the aqueous solution (C) is        separated from the suspension (B) in order to obtain an aqueous        suspension (D) comprising at least 18% and at most 50% of        calcium phosphate reactant particles, or to obtain a wet solid        (D′) comprising at least 50% and at most 80% of calcium        phosphate reactant particles, or to obtain a pulverulent solid        (D″) comprising at least 80% and at most 95% of calcium        phosphate reactant particles and at least 5% and at most 20% of        water.

Item 17. Particle of calcium phosphate reactant obtainable by theprocess according to item 13, comprising at least 60% by weight ofhydroxyapatite, and having a mean size of at least 30 μm.

Item 18. Particle of calcium phosphate reactant according to thepreceding item comprising at least 70%, preferably at least 75%, morepreferably at least 80% by weight of hydroxyapatite.

Item 19. Particle of calcium phosphate reactant according to Items 17 or18 having a mean size of at least 50 μm.

Item 20. Particle of calcium phosphate reactant according to items 17 to19 having a specific surface area of at least 50 m²/g, more preferablyof at least 110 m²/g, even more preferably of at least 120 m²/g, or ofat least 140 m²/g, or at least 160 m²/g.

Item 21. Particle of calcium phosphate reactant according to Items 17 or19 having a mean size of at most 200 μm, preferably of at most 100 μm,more preferably of at most 70 μm.

Item 22. Particle of calcium phosphate reactant according to items 17 to21 comprising at least 2%, preferably at least 5%, and more preferablyat least 6% by weight of hydroxide ions.

Item 23. Particle of calcium phosphate reactant according to items 17 to22 covered with plate-like crystallites, and wherein the plate-likecrystallites have a thickness of at most 25 nm, preferably of at most 20nm, more preferably of at most 10 nm.

Item 24. Particle of calcium phosphate reactant according to item 23wherein the thickness of plate-like crystallites is at least 1 nm,preferably at least 2 nm, more preferably at least 4 nm.

Item 25. Particle of calcium phosphate reactant according to Items 17 to24 and having a mean size of at least 50 μm.

Item 26. Particle of calcium phosphate reactant according to Items 17 to25 wherein the solubilized phosphate of 10 g of such particles stirredin 90 mL of water during 24 hours with a lab magnetic barrel, thenfiltrated on a 0.45 μm nitrocellulose membrane, is less than 10 mg PO4/Lof water.

Item 27. Aqueous suspension (D) comprising at least 25% and at most 50%of calcium phosphate reactant particles according to item 17 to 26,preferably obtainable from the process of item 16.

Item 28. A pulverulent solid (D″) comprising at least 80% and at most95% of calcium phosphate reactant particles according to item 17 to 26and comprising at least 5%, preferably at least 6%, more preferably atleast 10%, even more preferably at least 15% of water, advantageouslyobtainable from the process of item 16.

Item 29. A pulverulent solid (D″) according to item 28 comprising atmost 20% of water.

Item 30. Particle of calcium phosphate reactant obtainable by theprocess according to item 14, comprising at least 60% by weight ofhydroxyapatite, and having a mean size of at most 30 μm, preferably ofat most 20 μm and having a specific surface area of at least 15 m²/g,preferably of at least 50 m²/g, and having a content of hydroxide ionsof greater than 2% by weight, preferably greater than 3.5% by weight,and more preferably greater than 4% by weight.

Item 31. Method for purifying a liquid effluent containing metallicelements such as: Al, Ag, Ba, Be, Ce, Co, Cd, Cu, Cr, Fe, Hg, La, Li,Mo, Ni, Pb, Pd, Rb, Sb, Sn, Th, Ti, U, V, Y, Zn and/or non-metallicelements such as As, B, F, Se, according to which the calcium phosphatereactant obtained by the process of any one of items 1 to 16 or thecalcium phosphate reactant of items 17 to 26, or the aqueous suspension(D) of item 27, or the pulverulent solid (D″) of items 28 to 29, ismixed into the liquid effluent for a sufficient time so that the calciumphosphate reactant absorbs at least a portion of the metallic and/ornon-metallic elements and the mixture is subjected to a clarification inorder to produce a liquid partially purified of metallic and/ornon-metallic elements, on the one hand, and the calcium phosphatereactant loaded with metallic and/or non-metallic elements that isremoved.

Item 32. Purification method according to item 31, in which the calciumphosphate reactant or the aqueous suspension (D), or the pulverulentsolid (D″) is used with the liquid effluent in a sludge blanket contactreactor;

with a contact time between the calcium phosphate reactant and theliquid effluent of at least 15 minutes;

and in which said sludge blanket contact reactor, the calcium phosphatereactant is present at a weight concentration of at least 0.5% byweight;

a liquid is recovered as overflow from the sludge blanket reactor;

a flocculant is added to the recovered liquid in order to form a mixturecomprising particles of calcium phosphate reactant entrained out of thecontact reactor and flocculated;

said mixture is then introduced into a settling tank where the mixtureis separated into:

-   -   the liquid partially purified of metallic elements and/or of        non-metallic elements, and said liquid is recovered as overflow        from the settling tank,    -   and into an underflow from the settling tank comprising        flocculated and settled particles of calcium phosphate reactant        recovered as underflow from the settling tank;

and at least one portion of the underflow from the settling tankcontaining flocculated and settled particles of calcium phosphatereactant is recycled to the sludge blanket contact reactor.

Item 33. Purification method according to items 31 or 32, wherein theclarification comprises the use of an anionic polyacrylamide, preferablyanionic polyacrylamide of molecular weight from 5.10⁶ to 20.10⁶ daltonand from 2 to 50% anionicity in mole %, or more preferably anionicpolyacrylamide of molecular weight of 5.10⁶ to 15.10⁶ dalton and from 5to 15% anionicity in mole %.

Item 34. Purification method according to items 31 or 32, wherein theclarification comprises the use of an anionic modified starch,preferably derived from: potato starch, or corn starch, or tapiocastarch.

Item 35. Method for purifying a liquid effluent containing organiccompounds such as organic compounds present in black waters from an H₂O₂manufacturing process, in particular: Phthalic acid, 4-tertiary amylphthalic acid, 4-secondary amyl phthalic acid, Amylphtalic acidderivatives, Aromatic compounds, 4-tertamyl anthraquinone (TAQ),4-sectamyl anthraquinone (SAQ), according to which the calcium phosphatereactant obtained by the process of any one of items 1 to 16 or thecalcium phosphate reactant of items 17 to 26, or the aqueous suspension(D) of item 27, or the pulverulent solid (D″) of items 28 to 29, ismixed into the liquid effluent for a sufficient time so that the calciumphosphate reactant absorbs at least a portion of the organic compoundsand the mixture is subjected to a clarification in order to produce aliquid partially purified in organic compounds, on the one hand, and thecalcium phosphate reactant loaded with organic compounds that isremoved.

The examples, the description of which follows, serve to illustrate theinvention.

In these examples the pH measurements were made using a WTW Sentix 41electrode (pH 0-14, temperature: 0° C.-80° C.), a pH meter WTW pH3110.The calibration of the equipment was made using three buffer solutions:at pH 4.0 (batch Dulco test-0032) Prominent, a WTW pH 7.0 (WTW D-82362)and at pH 10.01 Hach (cat 27702). Note: If multiple sample measurementswere to be made with the same electrode, the electrode was rinsed withdeionized water between each measurement.

The measurement of the residual water was performed using an infraredanalyser Ref MA150C from Sartorius. For this, 10 to 20 g of product aredried at 105° C. till a constant weight is obtained during at least 5minutes.

The particle size measurement was carried out on a Beckman Coulter LS230 laser diffraction particle size analyser (laser of wavelength 750nm) on particles suspended in water and using a size distributioncalculation based on Fraunhofer diffraction theory (particles greaterthan 10 μm) and on Mie scattering theory (particles less than 10 μm),the particles being considered to be spherical.

The BET specific surface area was determined by gas adsorption on aMicromeritics ASAP2020 machine. Before the analysis, the samples (0.7 to1 g) are pretreated under vacuum at 250° C. until a stable vacuum of 4-5μbar has been achieved. The measurements were carried out using nitrogenas adsorbent gas at 77° K via the volumetric method, according to theISO 9277: 2010 standard (Determination of the specific surface area ofsolids by gas adsorption—BET method). The BET specific surface area wascalculated in a relative pressure (P/P0) range varying from around 0.05to 0.20.

In the examples, the heavy metal contents of the substances in the solidstate were measured in the following manner. A sample of the substanceto be analysed (around 200 mg) had 1 ml of 65% Suprapur nitric acid and3 ml of 37% Suprapur hydrochloric acid added to it, then it wassubjected to microwave radiation in a hermetically-sealed container inorder to mineralize it (destruction of the (hydro)carbon matrix in orderto obtain a mineral residue containing the analytes). This solution,with the addition of an internal standard (scandium) was made up tovolume (50 ml) with ultrapure water. The solution to be analysed wasthen filtered if necessary (0.45 μm) and injected in the form of anaerosol into an argon plasma sustained by inductive coupling. Attemperatures of 6000 to 8000° K, the atoms and ions formed from thesample were excited and returned to the ground state by emittingradiation with wavelengths characteristic of the element in theUV/visible range (130 nm to 800 nm) (ICP OES).

The various radiation wavelengths were separated by diffraction on agrating having a large number of lines and the intensity of the chosenlines was measured. The concentration of the target elements in themeasurement solution was obtained after calibrating the instrument withsolutions of known concentrations of each of the target elements. Theconcentration of these elements in the starting sample was thencalculated taking into account the dilution performed during the variouspreparation steps.

In the examples, the metal activity test (also called metal trappingactivity or metal adsorption activity) is measured according thefollowing operating procedure: An activity test needs at least 1 litreof stock solution; for this: Prepare more than one litter stocksolutions of 200 mg/l of the metal to be tested (for Cu, Ni Zn, thefollowing soluble metal salts may be used respectively: CuSO₄.5H₂O,NiCl₂, ZnCl₂), measure the initial metal concentration withspectrophotometric kits hach-lange after a dilution of 50 times (ref:Cu-LCK329, Ni-LCK337, Zn-LCK360). Measure the dry matter (DM) of apatiteby using an infrared analyser type MA150C by Sartorius. Introduce theapatite, equivalent 100% dry matter, (respectively 1 g DM for zincactivity, 1.5 g DM for copper activity and 5 g DM for nickel activity)in bottles of 1 liter, filled with 1 liter of the corresponding stocksolution. Shake the bottles for 3 hours at 10 rpm, with a lab rotatingmachine (such as the ones used on TCLP test). Sample 10 ml of thesolution with a syringe and filter it with a Millipore filter (0.45 μm),recover the filtrate into a test tube. Take precisely 1 ml of solutionin the test tube and perform a precise dilution of 50. Residual metalsconcentration are analyzed with spectrophotometric kits hach-lange (ref:Cu-LCK329, Ni-LCK337, Zn-LCK360). After correction of dilution factors,the quantity of adsorbed metal is calculated by difference of theinitial concentration and the residual one.

Example 1 In Accordance with the Invention

500 g of limestone in powder form having a D50 of less than 300 μm aredispersed in 2469 ml of water at a temperature of 20° C. 646 g ofphosphoric acid (H₃PO₄) having a weight concentration of 75% are addedto this suspension, over a duration of 20 minutes using a peristalticpump. This addition is accompanied by a temperature increase of 13° C.The mixture continues to be stirred vigorously using a stirrer having 4inclined blades at 700 rpm (power dissipated by the stirrer of the orderof 1 kW/m³), ensuring a good mixing of the sources of calcium andphosphate ions, and making it possible to degas the carbon dioxideformed during the addition of acid. At the end of the addition of acid,the suspension is then heated to 60° C. Once the suspension is at thistemperature, a 25 wt % suspension of Ca(OH)₂ (244 g of Ca(OH)₂ suspendedin 732 g of water) is added using a peristaltic pump in order tomaintain the pH of the suspension at 10±0.5 for 60 minutes. Once the 60minutes have passed, the heating is stopped and the stirring iscontinued but reduced to 50% rotational speed and the suspension is leftto cool for about 10 hours until it returns to ambient temperature (22°C.).

The calcium phosphate reactant particles of the suspension have, ascomposition: 94% by weight of hydroxyapatite, 5% by weight of calciumcarbonate, and comprising 3.5% of hydroxide ions. The physicalcharacteristics of the calcium phosphate reactant particles obtained aregiven in Example 3.

Example 1a In Accordance with the Invention

500 g of limestone are dispersed in 2469 ml of water at 50° C. 646 g of(75%) H₃PO₄ are then added to this suspension, over 20 minutes using aperistaltic pump. The mixture continues to be stirred vigorously at 700rpm (1 kW/m³) using a 4-blade stirrer, enabling the mixing of thecompounds and making it possible to degas the carbon dioxide formedduring this step. At the end of the addition of acid, the mixture isstirred for 30 minutes. A 25 wt % suspension of Ca(OH)₂ (250 g ofCa(OH)₂ suspended in 750 g of water) is then added using a peristalticpump in order to maintain the pH at 10±0.5 for 60 minutes. Once the 60minutes have passed, the heating is stopped and the stirring iscontinued but reduced to 50% of its power and the suspension is left tocool for about 10 hours until it returns to ambient temperature (22°C.).

The calcium phosphate reactant particles of the suspension have, ascomposition: 95% by weight of hydroxyapatite, 4% by weight of calciumcarbonate, and comprising 3.6% of hydroxide ions. The physicalcharacteristics of the calcium phosphate reactant particles obtained aregiven in Example 5.

Example 1b In Accordance with the Invention

Calcium phosphate reactant particles were prepared in the sameconditions as the ones of example 1, except at the end of the additionof acid, the suspension is then heated to 50° C., and once thesuspension is at this temperature, the 25 wt % suspension of Ca(OH)₂ isadded to maintain the pH of the suspension at 9±0.5 for 60 minutes. Oncethe 60 minutes have passed, the heating is stopped and the stirring iscontinued but reduced to 50% rotational speed and the suspension is leftto cool for about 10 hours until it returns to ambient temperature (22°C.) as example 1.

The calcium phosphate reactant particles of the suspension have, ascomposition: 94% by weight of hydroxyapatite, 5% by weight of calciumcarbonate, and comprising 3.5% of hydroxide ions. The final density ofsolid suspension in aqueous suspension (B) was 18% in weight (solidweight reported to total weight of the aqueous suspension). The physicalcharacteristics of the calcium phosphate reactant particles obtained aregiven in Example 6.

Example 1c In Accordance with the Invention

Influence of the solid suspension density during the manufacturingprocess. Calcium phosphate reactant particles were prepared in the sameconditions as the ones of example 1b, except that the final density ofsolid suspension in the slurry targeted and achieved was 5% by weight,and for this an initial quantity of water used was 14205 mL (and not2469 mL).

The calcium phosphate reactant particles of the suspension have, ascomposition: 94% by weight of hydroxyapatite, 5% by weight of calciumcarbonate, and comprising 3.5% of hydroxide ions. The physicalcharacteristics of the calcium phosphate reactant particles obtained aregiven in Example 6.

Example 1d In Accordance with the Invention

Influence of mixing energy during the manufacturing process. Calciumphosphate reactant particles were prepared in the same conditions as theones of example 1b, except that the reactor was mixed with at a lowerrotational speed at 320 rpm (corresponding to a power reported to volumeof reactor of P/V: ±0.2 kW/m³) (as in example 1b the rotational speedand energy of mixing reported to the volume of reactor were respectively700 rpm and 1 kW/m³).

The calcium phosphate reactant particles of the suspension have, asimilar chemical composition of the one of example 1b. The calciumphosphate reactant particles have a D50 of 73 μm and a specific surfacearea of 166 m²/g.

Example 2 In Accordance with the Invention

100 g of limestone are dispersed in 148 ml of water at ambienttemperature of 20° C. This suspension is then poured over a solution of71 g of NaH₂PO₄ in 166 ml of water at ambient temperature of 20° C. Themixture continues to be stirred vigorously, enabling the mixing of thecompounds and the degassing of the carbon dioxide formed during thisstep, over 30 minutes.

Once the addition is completed, a solution of 43 g of CaCl₂.2H₂O in 44 gof water is added over 5 minutes in order to convert the solublephosphates that have not yet reacted. Next, 58 g of a 25 wt % suspensionof lime are added with vigorous stirring using a stirrer having 4inclined blades.

Once the entire suspension of lime has been introduced, the stirringspeed is reduced to 50% and the suspension is stirred for 10 hours.

Example 3 In Accordance

The suspension obtained in Example 1 consists of calcium phosphatereactant particles having a D50 of 61 μm and a specific surface area of110 m²/g suspended in an aqueous solution.

The suspension is then centrifuged in order to obtain a wet solidcontaining 55%±5% dry matter.

Example 4 In Accordance

The wet solid obtained in Example 3 is then dried in a ventilatedchamber at a temperature of 40° C.±5° C. until a solid is obtained thatcontains 94% dry matter, formed of a powder of particles having aparticle size D50 of 61 μm and a specific surface area of 110 m²/g.

Example 5 In Accordance

The suspension obtained in Example 1a is centrifuged in order to obtaina wet solid containing 55%±5% dry matter; the solid is formed ofparticles having a D50 of 10 μm and a specific surface area of 16 m²/g.

Example 6 In Accordance

The suspensions obtained in Examples 1b & 1c consists of calciumphosphate reactant particles which specific surface area (S BET) and D50are listed in tab. 1.

The suspensions of examples 1b and 1c where then centrifuged to obtain awet solid containing 55%±5% dry matter.

TABLE 1 Reactant particles S BET and D50 from examples 1b & 1c. Ca/PReactant S BET D(50) from example 1b 166 m²/g 54 μm 18% weight fromexample 1c 180 m²/g 20 μm 5% weight

The corresponding calcium-phosphate reactants were evaluated regardingthe metal trapping activity for Zn, Cu, Ni. Results are given in table2. The figures show that even with a higher specific surface obtained atlower density of solid suspension during the manufacturing of the Ca/Preactant, the metal trapping activity of the reactant from example 1c islower than the activity of reactant from example 1b, showing no interestto lower the density of suspension during steps 1 and 2.

TABLE 2 Reactant particles metal trapping activity from examples 1b & 1cmg metal/g of Ca/P Reactant Ca/P Reactant Zn Cu Ni from example 1b 119128 26 18% weight from example 1c 103 112 24 5% weight

Example 7 In Accordance

Impact of Water Content on Metals Trapping Activity

A more important quantity of Ca/P Reactant particles were prepared usingsame operating conditions as example 1.b but using and increased size ofreactor (useful volume of 200 L) and correspondingly increasing thequantity of raw materials. The obtained Calcium phosphate Reactantparticles were dried in a ventilated oven with varying drying time sothat to obtain four different solids, with four different residual watercontents, respectively with: 50%, 17%, 6% and less than 1% residualwater. The ability to trap metals (Cu, Ni and Zn), was measured for eachobtained solid (wet solid with 50% water content, or pulverulent solidwith 17%, 6% or <1% water content). The results are detailed in thetable below, expressed in quantity of metal (mg) trapped by quantity ofdry Ca/P Reactant (g).

TABLE 3 Reactant particles metal trapping activity from example 7 mgmetal/ g Ca/P Activity Reactant loss Ca/P Reactant Zn Cu Ni Zn Cu Ni 50%water 99 109 20 — — — 17% water 75 86 17 24% 21% 14%  6% water 81 77 1717% 30% 14% <1% water 68 62 14 31% 44% 29%

One can clearly observe that calcium-phosphate reactant has a decreasedmetal trapping activity when dried, and that a Ca/P reactant comprisingmore than 5% by weight of water (or at least 6% by weight of water)shows limited and acceptable metal trapping activity. As Ca/P reactantcomprising 5% by weight of water or less, shows up to 33 to 41% loss ofmetal trapping activity for Zn and Cu.

Example 8 In Accordance

Impact of Drying Conditions on Metal Trapping Activity

The Ca/P reactant was dried using two different methods, the first onein a ventilated oven at 80° C., the second one at ambient temperature(AT) at about 20 to 22° C., though to obtain the same targeted andobtained residual water content in the solids, respectively about 17 to20% and about 5 to 6%. Then, their ability to trap metals was checkedand evaluated on Zn, Cu, Ni. The results detailed in table 4 andexpressed in quantity of metal (mg) trapped by quantity of dry apatite(g) demonstrate that, what is crucial for activity is the residual watercontent, and not the way the reactant particles are dried.

TABLE 4 Reactant particles metal trapping activity according dryingmethod mg metal/g Ca/P Activity Drying water Reactant loss method [%] ZnCu Ni Zn Cu Ni AT 17 75 86 17 — — — Oven (80° C.) 20 87 81 17  0% 5% 1%AT 6 81 77 17 — — — Oven (80° C.) 5 66 70 16 19% 9% 7%and residual water content of the Ca/P Reactant.

Example 9 Not in Accordance with Present Invention

Comparison of Ca/P Reactant of present invention with apatite frompatent application DE10330689A1. Example 3 (on §[0022] and following §)from DE10330689A1 was reproduced as described here after, to determinethe specific surface of the reactant obtained, as the document is silenton this characteristic.

In a 5 l reactor, 3 l of water were introduced, then 111 g of Ca(OH)₂was added and maintained in suspension with a 4 blades inclinedagitator, providing 0.2 W/l (or kW/m³) of stirring power. 104 g ofphosphoric acid at 85% was then added in the reactor in 1 h, using aperistaltic pump. The pH of the reactor, during the addition ofphosphoric acid, is maintained at a minimum of 9 with a solution ofcaustic soda 1M if necessary. Once all the phosphoric acid added and thepH stabilized at about 9, the content of the reactor is filtered throughpaper filter and dried in an oven at 80° C. for 24 h. As described inDE10330689A1, the obtained solid after 24 h was a block of solid thatcan be grinded in granules of 1 to 8 mm.

So before drying the product, part of the suspension was sampled inorder to characterize the calcium phosphate reactant particles thussynthesized before drying. The reactant particles had a D50 of 30 μm anda specific surface area of 109 m²/g (see Tab. 5 for a comparison withCa/P Reactant in accordance to present invention).

By comparison the reactant particles dried in same condition gives apulverulent powder as the reactant powder of DE10330689A1 when driedgives a compact block.

Pictures on SEM microscope, given on FIGS. 2 & 4, show that the surfaceof the obtained reactant particles is covered with needles-likecrystallites.

For comparison, pictures on SEM microscope, given on FIGS. 1 & 3, showthat the obtained reactant particles from present invention (example 1b)are covered with plate-like crystallites.

TABLE 5 Reactant particles S BET and D50 from example 1b (in accordancewith present invention) and from DE10330689A1 (not in accordance). Ca/PReactant S BET D(50) from example 1b 166 m²/g 54 μm 18% weight fromDE10330689 109 m²/g 30 μm

Moreover the Reactant particles metal trapping activity fromDE10330689A1 (not in accordance) show low values compared to Reactantparticles in accordance to present invention: see Tab. 6.

TABLE 6 Reactant particles metal trapping activity from Reactant inaccordance with present invention and from DE10330689A1 (not inaccordance). mg metal/g Reactant Ca/P Reactant Zn Cu Ni from presentinvention 113 126 25 from DE10330689 55 56 13

Also the measure of the solubilized phosphate of the reactant fromDE10330689A1 (solubilized phosphate of 10 g of reactant particlesstirred in 90 mL of water during 24 hours with a lab magnetic barrel,then filtrated on a 0.45 μm nitrocellulose membrane) is:

14 mg PO4/L for the DE10330689A1 reactant

4 mg PO4/L for typical reactant obtained by present invention.

Example 10 In Accordance

The suspension obtained in Example 1 is used in a 20 litretransparent-walled reactor. The reactor is equipped with a stirrerhaving 4 inclined blades rotating at around 75 rpm, and the rotationspeed of which is adjusted in order to keep the solid in suspension andto form in the reactor a sludge blanket that is visible to the nakedeye, occupying 80%±5% of the volume of the reactor, in the bottom partof the reactor. The reactor is fed continuously with liquid effluent ata flow rate of 25 l/h at the base of the sludge blanket reactor. Addedto the overflow from the reactor is a flocculant of polyacrylamide typeRef Floerger AN905SH, with a controlled flow rate of the order of 200ml/h in order to obtain 1 ppm of flocculant with respect to the watersat the outlet of the sludge reactor. The overflow from the reactor andthe flocculant supplying the base of a settling tank having a surfacearea of 0.0154 m² make it possible to obtain a rate of rise of thetreated waters of 1.3 m/h. The treated waters are recovered as overflowfrom the settling tank, and the sludges settled at the base of thesettling tank are reintroduced semi-continuously into the sludge blanketreactor with a pump operating at a flow rate of 6 l/h.

Such a process has shown to be effective for at least the followingsoluble metallic elements: Al, Ag, Ba, Be, Ce, Co, Cd, Cu, Cr, Fe, Hg,La, Li, Mo, Ni, Pb, Pd, Rb, Sb, Sn, Th, Ti, U, V, Y, Zn and/ornon-metallic elements such as As, B, F, Se.

Different anionic flocculants of polyacrylamide type (from SNF Floerger:AN905VHM, AN910SH, AN 934, AN 934MPM, AN934SH, AN934VHM, AN945SH,AN977SH, EM532) or of anionic modified starch (Biosourced: Hydrex) frommolecular weight from 5.10⁶ to 20.10⁶ dalton and from 2 to 50%anionicity in mole %, or of non-ionic type (from SNF Floerger FA920SH),were tested. The best results are obtained using anionic flocculentssuch as of polyacrylamide type of 5.10⁶ to 15.10⁶ dalton molecularweight and 5 to 15% anionicity in mole %, or using anionic starch ofHydrex type. Concentration of 1 to 6 ppm in solution, preferably 3 to 5ppm, gave the best sedimentation time for 2 to 5% dry matter of densityof suspension of solids.

Example 11 In Accordance

The solid obtained in Example 4 is used in a fluidized bed, composed ofa column, having a diameter of 6 cm and a volume of 2 litres. Thefluidized bed is supplied at its base with the aqueous effluent to betreated and with a liquid shuttle (cf. infra).

The calcium phosphate reactant containing apatite is kept in suspensionin the fluidized bed by means of a liquid shuttle of 29 l/h produced bya recirculating pump operating on a 3 l buffer tank supplied by theoverflow from the fluidized bed. The system ensures a minimum contacttime of 15 minutes between the calcium phosphate reactant and theeffluent to be treated. A flocculant, of SNF Floerger AN905VHM type, isinjected at a flow rate of 120 ml/h into the 29 l/h shuttle before entryinto the fluidized bed in order to ensure a concentration of the orderof 1 ppm in the aqueous effluent to be treated.

The treated aqueous effluent is extracted by overflow from the bufferreactor at the outlet of the fluidized bed.

Example 12 In Accordance

Example for in-Line Injection:

A tube of 10 m length and 14 mm of internal diameter curved to have 5portions of 1.8 m length, the remaining length of the tube being thecurved sections, is used. 200 l of a synthetic solution containing about300 μg/l of each of the three metals, Cu, Zn et Ni, is then injected ata flow rate of 50 l/h and an aqueous suspension of 1 l containing 5 g ofapatite is co-injected at the beginning of the tube equipment at a flowrate of about 250 ml/h, so that to maintain a contact time of about 1minute. Samples of treated water solution are analyzed at the outlet ofthe system and metals concentrations are analyzed withspectrophotometric kits hach-lange (ref: Cu-LCK529, Ni-LCK537,Zn-LCS360). The concentrations of the three heavy metals have beendecreased below the quantification of the tests, respectively: 10 μg/lfor Cu, 20 μg/l for Zn and 50 μg/l for Ni).

This test shows the excellent reactivity of the Reactant accordingpresent invention, even at short contact time of about 1 minute.

Example 13 In Accordance

Example for Organic Molecules Adsorption:

1 l of a solution of Dark effluent waters, from an H₂O₂ manufacturingprocess, was contacted with 50 g of calcium phosphate reactant particlesaccording example 1b, and was stirred with a laboratory magneticstirrer. A sample of the solution was sampled after one hour of contactwith the Reactant particles, filtered through a 0.45 μm membrane andthen analyzed by HPLC to assay the remaining compounds. The results ofinitial organic molecules concentration and final concentrations aftercontact with the Reactant and filtration are shown on table 7.

TABLE 7 Organic molecules concentration in initial black waters beforecontact and after one hour of contact with Ca/P Reactant particles andcorresponding removal efficacy of said organic compounds. after 1 hbefore contact and removal contact separation efficacy Compound [mg/l][mg/l] % Phthalic acid 42 23 45 4-tertiary amyl 599 256 57 phthalic acid4-secondary amyl 454 159 65 phthalic acid Amylphtalic acid 127 57 55derivatives Aromatic compounds 266 168 37 4-tertamyl 1.9 0 100anthraquinone (TAQ) 4-sectamyl 1.8 0 100 anthraquinone (SAQ)

Should the disclosure of any patents, patent applications, andpublications which are incorporated herein by reference conflict withthe description of the present application to the extent that it mayrender a term unclear, the present description shall take precedence.

1. A process for producing a calcium phosphate reactant, according towhich: in a first step, using a source of calcium and a source ofphosphate ions in water, in a molar ratio that is adjusted to obtain aCa/P molar ratio of between 0.5 and 1.6, and reacting the source ofcalcium with the phosphate ions at a pH of between 2 and 8, in order toobtain a suspension (A) of calcium phosphate, and in a second step,adding to the suspension (A) an alkaline compound comprising hydroxideions in order to set a pH of more than 8, and an additional source ofcalcium to obtain a suspension (B) of calcium phosphate reactant havinga Ca/P molar ratio of more than 1.6.
 2. The process according to claim1, wherein the source of calcium comprises calcium carbonate, calciumoxide, calcium hydroxide, calcium chloride, calcium nitrate, or calciumacetate.
 3. The process according to claim 1, wherein the source ofphosphate ions is phosphoric acid.
 4. The process according to claim 1,wherein, in the first step, the Ca/P molar ratio is: between 0.50 and1.35, or: between 1.4 and 1.6.
 5. The process according to claim 1,wherein stirring of the suspension (A) and the density of suspension, inthe second step, are adjusted in order to avoid appearance of a calciumphosphate gel having a viscosity of at least 200 cps.
 6. The processaccording to claim 1, wherein, in the second step, the alkaline compoundused that comprises hydroxide ions is selected from the group consistingof sodium hydroxide, calcium hydroxide, and combination.
 7. The processaccording to claim 1, wherein, in the second step, the additional sourceof calcium is selected from the group consisting of calcium chloride,calcium nitrate, and calcium acetate, and wherein the additional sourceof calcium is added in addition to the alkaline compound, in order tofinely adjust the Ca/P ratio in the second step and limit theconcentration of phosphorus element in an aqueous solution (C) of thesuspension (B) to at most 5 mmol of phosphorus element per liter of theaqueous solution (C).
 8. The process according to claim 1, wherein thefirst step is carried out at a temperature of less than 50° C.
 9. Theprocess according to claim 1, wherein the first step is carried out at atemperature of at least 50° C.
 10. The process according to claim 1,wherein the second step is carried out at a temperature of at least 40°C.
 11. Particle of calcium phosphate reactant, comprising at least 60%by weight of hydroxyapatite, from 5% to 20% by weight of calciumcarbonate, at least 5% by weight water, said particle having a mean sizeof at least 30 μm, and having a specific surface area of at least 110m²/g.
 12. A method for purifying a liquid effluent containing metallicelements and/or non-metallic elements comprising: mixing particlesaccording to claim 11 into the liquid effluent to form a mixture for asufficient time so that the calcium phosphate reactant particles absorbat least a portion of the metallic and/or non-metallic elements; andsubjecting the mixture to a clarification to produce a liquid partiallypurified of metallic and/or non-metallic elements and the calciumphosphate reactant particles which are loaded with metallic and/ornon-metallic elements and that are removed.
 13. The purification methodaccording to claim 12, wherein the calcium phosphate reactant particlesare used with the liquid effluent in a sludge blanket contact reactor;with a contact time between the calcium phosphate reactant particles andthe liquid effluent of at least 1 minute; wherein in said sludge blanketcontact reactor, the calcium phosphate reactant particles are present ata weight concentration of at least 0.5% by weight; wherein a liquid isrecovered as overflow from the sludge blanket reactor; wherein aflocculant is added to the recovered liquid in order to form a mixturecomprising particles of calcium phosphate reactant entrained out of thecontact reactor and flocculated; wherein said mixture is then introducedinto a settling tank where the mixture is separated into: the liquidpartially purified of metallic elements and/or of non-metallic elements,said partially purified liquid being recovered as overflow from thesettling tank, and into an underflow from the settling tank, saidunderflow comprising flocculated and settled particles of calciumphosphate reactant; and wherein at least one portion of the underflowrecovered from the settling tank and containing flocculated and settledparticles of calcium phosphate reactant is recycled to the sludgeblanket contact reactor.
 14. The particle of calcium phosphate reactantaccording to claim 11, having a specific surface area of at least 120m²/g.
 15. The particle of calcium phosphate reactant according to claim11, being covered with plate-like crystallites, wherein the plate-likecrystallites have a thickness of at most 25 nm or of at least 1 nm. 16.The particle of calcium phosphate reactant according to claim 11,comprising at least 6% by weight of water.
 17. The particle of calciumphosphate reactant according to claim 11, comprising from 5% to 20%water.
 18. The particle of calcium phosphate reactant according to claim11, further comprising calcium hydroxide.
 19. A pulverulent solidcomprising: at least 80% and at most 95% of calcium phosphate reactantparticles, and at least 5% and at most 20% by weight of water, saidcalcium phosphate reactant particles comprising at least 60% by weightof hydroxyapatite and from 5% to 20% by weight of calcium carbonate,said calcium phosphate reactant particles having a mean size of at least30 μm and having a specific surface area of at least 110 m²/g.
 20. Thepulverulent solid according to claim 19, wherein the calcium phosphatereactant particles further comprise calcium hydroxide.